1. Field of the Invention
The present invention relates to an information recording apparatus, such as a computer data storage system, which is capable of recording information on magnetic tape.
2. Description of Related Art
Magnetic tape has a variety of uses. For example, it can be used as audiotape, videotape, computer tape, and the like. In the field of data backup tape in particular, with an increase in the capacity of hard drives to be backed up, magnetic tape cassettes with a recording capacity of several terabites per reel have been commercialized. In order to keep up with a further increase in the capacity of hard drives in the future, an increase in the capacity of backup tape is essential.
Methods of increasing the capacity of magnetic tape include, for example, reducing the thickness of the magnetic tape. According to such a method, the recording capacity can be increased by increasing a per-reel tape length, without increasing the roll diameter of the magnetic tape around the reel.
On the other hand, information recording apparatuses carry out tracking servo control so that a magnetic head can trace a target track of the magnetic tape. Examples of tracking servo systems include a magnetic servo system and an optical servo system. In the magnetic servo system, servo tracking is carried out by magnetically recording servo signals in a magnetic layer of the magnetic tape and magnetically reading the servo signals. In the optical servo system, servo tracking is carried out by forming recessed arrays (servo signals) in a back-coat layer of the magnetic tape using laser irradiation or the like and optically reading the recessed arrays.
When recording data on or reproducing data from the magnetic tape using these servo systems, it is possible to allow the magnetic head to trace the recording track even if the position of the magnetic tape changes in the width direction relative to the magnetic head.
Specifically, first, servo signals recorded on the magnetic tape are read by a servo head. Next, a position error signal (PES) is calculated on the basis of the read-out servo signals. Then, by controlling the position of a magnetic head unit (including the servo head, a data recording head, and a data reproduction head) in the width direction of the magnetic tape based on the calculated PES, the data recording head or the data reproduction head is allowed to trace the target data track. As a result, information can be recorded on the magnetic tape with precision and information recorded on the magnetic tape can be reproduced with precision.
JP H8-30942 A discloses a timing-based servo system. In the timing-based servo system, servo signals are recorded on magnetic tape as a pattern that is inclined with respect to the width direction of the magnetic tape. In the timing-based servo system, the head position is recognized from a time interval between peaks of reproduced waveforms at the time of reproducing the servo signals.
However, in the convention configurations as described above, when the thickness of the magnetic tape is reduced to increase the recording capacity of the magnetic tape, the thickness of the magnetic layer becomes small. Thus, the servo signal output and the signal-to-noise ratio (S/N ratio) decline, and consequently, the precision of tracking servo control drops. For example, since a servo signal in conformity with the Linear Tape Open (LTO) standard has a fairly long wavelength, the servo signal output and the S/N ratio decline when the thickness of the magnetic layer becomes small.